This invention relates to measurements of the diameter of an optical fiber, and, more particularly, to an apparatus and method for performing such measurements along the length of a rapidly moving optical fiber.
Glass fibers for optical information transmission are strands of glass fiber processed so that light transmitted through the fiber is subject to total internal reflection. A large fraction of the incident intensity of light directed into the glass fiber is received at the other end of the fiber, even though the glass fiber may be hundreds or thousands of meters long. Optical-quality glass fibers have shown great promise in communications applications, because a high density of information may be carried along the glass fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a plentiful substance, silicon dioxide.
The glass fibers are fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical glass fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage, and the resulting coated glass fiber is generally termed an "optical fiber" in the art. As an example of the dimensions, in a typical configuration the diameter of the glass fiber is about 125 micrometers, and the diameter of the glass fiber plus the polymer buffer (the optical fiber) is about 250 micrometers (approximately 0.010 inches).
The optical fiber may be wound onto a cylindrical or tapered cylindrical bobbin with many turns adjacent to each other in a side by side fashion. After one layer is complete, another layer of fiber is laid on top of the first layer, and so on. The final assembly of the bobbin and the wound layers of optical fiber is termed a canister, and the mass of wound optical fiber is termed the fiber pack. When the optical fiber is later to be used, the optical fiber is dispensed from the canister in a direction generally parallel to the axis of the cylinder.
The preparation of a canister demands great care and precision in winding of the optical fiber. The velocity of the optical fiber as it is later unwound from the bobbin may be as high as several hundred meters per second. If any snags, uneven stresses, or other irregularities are present, they can cause the optical fiber to break.
One important manufacturing variable of the optical fiber is its outer diameter. Variation in the diameter can result from variation in either the diameter of the glass fiber or the thickness of the buffer layer, but more typically is due to the latter reason. Such variation in optical fiber diameter, if too great, can produce winding irregularities that can lead to breaking of the optical fiber as it is later unwound. Thus, for example, if the optical fiber in one layer is uniformly of one diameter, and then the optical fiber wound over it in the next layer is of a slightly larger diameter in one region, the large diameter in a portion of the second layer may result in overly high in-layer stresses and a "pop up" defect in that or a neighboring layer. Such a defect can result in snagging and breaking of the optical fiber as it is unwound from the bobbin.
One fiber break in a long optical fiber disrupts the communications through the optical fiber, which can lead to a disastrous and costly complete system failure. Even one out-of-tolerance diametral variation, over only a few centimeters of the length of an optical fiber that is many kilometers long, may therefore be unacceptable. It is therefore necessary in critical applications to inspect the diameter of the optical fiber to be certain that it meets tolerances over its entire length. In a production setting such an inspection is quite a demanding challenge, since a long length must be inspected.
In one possible approach, a length of optical fiber is progressively unwound from a source spool, transported past an inspection station, and then rewound onto a receiver spool. One candidate type of inspection station, which is now commercially available, is a laser scanner. A laser beam whose diameter is less than that of the optical fiber is rapidly scanned from side to side in a scanning pattern that is longer than the diameter of the optical fiber. A photocell behind the optical fiber measures the intensity of the light. As the laser beam first intercepts the optical fiber, the measured intensity drops, remains low as long as the laser beam intercepts the optical fiber, and then rises to the initial value when the laser beam no longer intercepts the optical fiber. A timer measures the length of time that the intensity is reduced, and from this time and the scanning speed the diameter of the optical fiber is calculated.
The laser scanning approach is operable when the optical fiber is stationary or moving slowly, but at higher translational speeds of several meters per second becomes unsatisfactory for several reasons. Scanning is typically accomplished by bouncing the laser beam against a rotating faceted mirror. The highest rotational rates of the mirror currently possible do not produce a sufficient scanning frequency to make measurements at the desired interval along a rapidly moving optical fiber as it moves past the measurement apparatus. The scan line travels across the moving optical fiber in a diagonal path, so that the diameter measurement is not at a single longitudinal location. Transverse vibration of the optical fiber can additionally interfere with the measurement. Moreover, the electronics cannot process the information at the required rates.
There is therefore a need for an improved approach for evaluating the diameter of a moving optical fiber to be certain that it is within preselected acceptable tolerance limits. The present invention fulfills this need, and further provides related advantages.